Heat lamps used for reheating polymer preforms (parisons) for the commercial manufacture of beverage bottles are typically quartz lamps having a broad light emission spectrum from 500 nm to greater than 1500 nm. The maximum light emission from quartz lamps occurs in the range of about 1100-1200 nm. Polyester, especially polyethylene terephthalate ("PET"), absorbs poorly in the region between 500 and 1400 nm. So in order to speed up the reheat step in bottle production, agents which absorb light in the 700-1200 nm range can be added to the polyester polymer.
A variety of black and gray body absorbing compounds have previously been used as reheat agents to improve the heat up characteristics of polyester under quartz lamps. However, these compounds impart color to polymers. Therefore, the amount of absorbing compounds that can be added to a polymer is limited by its impact on the visual properties of the polymer, such as transparency. Transparency is represented as "L*" in the Gardner Color System, with an L* of 100 representing 100% transparency and an L* of 0 representing 100% opacity. Generally, darker colored reheat agents can be added in only very small quantities because of their negative impact on L*.
Previously disclosed examples of useful polymer reheat agents include carbon black (U.S. Pat. No. 4,408,004) and reduced antimony metal (U.S. Pat. No. 5,419,936 and U.S. Pat. No. 5,529,744). Additionally, U.S. Pat. No. 4,420,581 and U.S. Pat. No. 4,250,078 disclose using red iron oxide as an infrared absorber in polyester containing green dye. The red tint added by the red iron oxide is not problematic in that use because the green dye masks the red tint. While colorless and green beverage bottles have been found to be commercially useful, red tinted bottles have not been marketable. Without the masking presence of a green dye, red iron oxide would cause an undesirable red hue to an otherwise colorless, or neutral hue, polyester bottle.
A more darkly colored absorbing compound generally improves heat up characteristics better than a relatively lighter absorbing compound. However, the more darkly colored absorbing compounds can only be added in very small quantities due to the larger negative impact on L*. For example, when carbon black, a very dark black compound, is added to PET in concentrations greater than a few ppm, bottles blown from that PET are very gray and dull in appearance. Reduced antimony metal can be present in PET in concentrations up to about 50 ppm without having an excessive negative impact on L* because reduced antimony is a gray metal which is much lighter in color than true black body absorbers like carbon black.
U.S. Pat. No. 4,481,314, discloses the use of certain anthraquinone type dyes for the purposes of improving reheat rates. However, these dyes have substantial absorbance in the visible spectrum, resulting in coloration of the polymer. In addition, their relatively low molar extinction coefficients (.epsilon.) (in the range of 20,000) require the use of relatively large amounts of the dye (20-100 ppm) to the polymer. At a concentration of 50 ppm, the reheat rate improvement was 7%. However, at these levels the polymer displays a light green color which is not suitable for producing clear, neutral hue bottles.
In light of the above, it would be desirable to have an infrared absorber material which can be added to a thermoplastic polymer in a concentration sufficient to effectively increase the reheat rate of the polymer by about 10 percent, yet without deleteriously affecting polymer L* as much as previously known effective polymer reheat agents.